The invention relates to a lockstitch sewing machine rotating looptaker, more particularly, to a bobbin thread pull-off for such a looptaker.
A lockstitch is formed by concatenation of an upper, or needle thread with a lower, or bobbin thread. A looptaker is provided to pick up a loop of the needle thread carried through the work material by the sewing needle, enlarge the loop and cast it about a thread carrying bobbin supported internally of the looptaker. A sewing machine take-up lever is provided for the needle thread, to supply the needle thread required for the enlarged loop, and to take up the excess needle thread when the loop has been cast about the bobbin. When the excess needle thread is taken up by the take-up lever, it carries with it a bobbin thread, about which it is now looped. In subsequent stitches, the bobbin thread extends from the stitching to the bobbin and the needle thread loop must pull against the bobbin thread to be drawn, by the take-up lever, to the work material. As the needle thread loop about the bobbin thread becomes smaller, there is greater resistance to relative movement of the threads, a condition which becomes more severe as machine speed is increased. Even at constant machine speed, this condition creates a difficulty in making uniformly set stitches, the difficulty being compounded by zig-zag or pattern stitching where thread demand is increased.
In the prior art, this condition has been alleviated by providing means for pulling off bobbin thread from the bobbin; to, in theory, remove bobbin inertia and thread tension as a factor in the thread lock-up, thereby permitting easier flow of the bobbin thread. Some prior art machines with bobbin thread lead directly below a feed dog, have accomplished this by utilizing part of the feed dog return motion to pull off bobbin thread from the bobbin, requiring, however, extra mechanism or other design restraints. In U.S. Pat. No. 3,693,565, there is disclosed a lockstitch looptaker with bobbin thread lead between a bobbin case and looptaker body, wherein frictional surfaces on the looptaker body periodically engage with the bobbin thread to perform a pull off function. It is also known to provide a looptaker having a portion thereof contoured in a fashion to provide a positive pull off for bobbin thread, and also biasing the bobbin thread lead to one side of the sewing needle path to prevent half hitching. In this latter looptaker, however, considerable care and expense is required to insure that the pull off portion thereof is properly formed yet will not fray or weaken the bobbin thread.
What is required is a bobbin thread pull-off associated with a looptaker to work in conjunction therewith, without the necessity for careful or expensive finishing operations to insure the proper functioning thereof.